Molding techniques or injection-molding methods are widely used for processing plastics. With them, many different articles can be produced, for example from thermoplastic or thermosetting materials. The techniques of transfer molding and compression molding are used particularly often.
In the case of compression molding, the molding material, which may for example take the form of a paste or a quantity of pellets and is generally preheated, is introduced into an open, usually heated mold cavity, which is subsequently closed. The molding material is pressed into the mold by the effect of a pressure and fills it completely. The molding material remains in the mold until it has completely cured.
In the case of transfer molding, on the other hand, a defined quantity of the molding material, typically a thermosetting material, is filled into the otherwise closed mold cavity through a gate. During transfer molding, the walls of the mold are typically heated to a temperature above the melting temperature of the molding material in order to obtain good flow properties of the material within the cavity.
A large number of different thermoplastic and thermosetting materials come into consideration both for compression molding and for transfer molding.
It is possible both in the case of compression molding and in the case of transfer molding to produce a large number of usually identical articles simultaneously if the mold has more than one mold cavity. In the case of transfer molding, the molding material is then usually introduced into the individual cavities simultaneously from the supply container through a branched feed line (sprue and runner).
In particular when thermosetting materials are used, which as a result of the contact with the hot mold crosslink to an increasing extent as they flow over an increasing distance, or highly viscous thermoplastic materials are used, in particular with a mold that is not heated, the formation of so-called voids (pockets of air or gas resembling bubbles or cracks) can cause flaws in the articles produced. Voids form in particular in regions within the mold cavity that are of a particularly intricate and capillary nature, and in cases where particularly small articles are produced.
Voids may represent blemishes, but may also lead to malfunctions of the article produced, for example in the case of semiconductor devices. Various efforts are therefore made to prevent the formation of voids.
For example, in the production of plastic packages for semiconductor devices by means of transfer molding, the air present in the mold cavity is removed through vents during the molding process. A negative pressure of approximately 100 mbar is thereby reached in the mold cavity. It is particularly favorable in this case if the vents are arranged in the wall of the mold cavity that lies opposite the gate, because in this case the air can be removed from the entire flow path of the molding material.
This so-called vacuum molding does improve the result of the molding process and reduces the probability of voids forming. However, in the production of articles with a geometry that is particularly intricate and with particularly small cavities of a capillary nature, such as for example plastic packages for semiconductor devices, the risk of voids forming is so great even in the case of vacuum molding that process parameters, such as for example the temperature of the molding material and of the mold cavity, and also the vacuum have to be controlled very precisely. It is problematical in this respect that, although constant control of the process parameters is necessary, there is no simple way of continuously monitoring the formation of voids in the mold cavity.
Generally, vacuum sensors are not used within the mold cavity. Monitoring of the vacuum in the mold cavity is therefore not possible. Since, however, even small leaks in the lines can lead to a relatively great increase in pressure in the mold cavity, and consequently have a great effect on the tendency for voids to form, a good vacuum must be ensured at all times if use of the vacuum is to achieve a reduction in the formation of voids.